OK, the drive back from St. Augustine with the new C4S was a pleasure and got out of the car like a young guy, so that was fun. The fuel storage confused me, though. The '06 C4 had a 17Gal tank, and I expected the same from this '07. The drive to St. Augustine was 445 miles and I added 2 gallons from the gas can I'd carried and made it to the dealer. Speeds were in the 70s due to heavy fog most of the drive, and MPG was about 22.4, according to the trip computer.

The drive back was mid-80s driving, computer showed 19.8MPG, but I added 5 gallons and the computer showed 13 miles to empty by the time I hit BucEEs...almost bone dry and the red light had been on for almost 40 miles. Went to fill it up and it would only take 13.9 Gallons........WTH?

Gas gauge shows Full. Miles to empty were 399, so all of that seemed correct, but I couldn't put any more fuel into the tank.

?1. Could it be possible that the tank is only a 14G tank?

?2 If it's a 17G tank, how is it possible that all the gauges seemed to be working properly, except that there was an extra 3 gallons in the tank instead of it being empty?

?3 If I test the car and run it down to empty and below, it will either continue to run because of the extra fuel, or the car will die? If add fuel to the tank after the car has starved itself, will the car start like the old carbed cars, or will it take some special trick?

Weird question, but when the car is telling me one thing and the spec sheet is telling me another, is there any form of Reset to do?

 

It's a 16.9 gallon tank.

Did you try pumping past the shutoff at 13.9 gallons?

 

Sorry if you are already aware of the following ...

The fuel tank is convoluted on the AWDs. It must wrap around the extra drive parts and still be larger to compensate for the generally lower mpgs compared the 2WD cars. Thus the fill path is convoluted as well. It is, in general, a matter of flipping the fill handle upside down as you pump the gas. This is especially true if you are dealing with vapor capture systems. Regardless of how I fill, some pumps will shut off three gallons early anyway.

As far as the fuel gauge goes, this could be a calibration factor. I've had many cars where the gauge seems to go down very slowly for the top quarter tank. But my 2010 is not that way so what I just wrote is pure guessing.

 

You carry a gas can around with you?

 

I always fill up with the pump upside down to ensure it doesn’t hit the automatic cutoff too early due to vapor guards.

 

I never knew that, and will do so in the future. The C4 had no issues, and what's confusing is that, Yes, I tried placing more fuel while giving the tank time to burp, but it was saying FULL, and more confusing is that the gauge read past the F mark, not at 3/4.

I'll try adding from the 2-gallon gas can here at home and see if it takes it. If not, then the gauges and computer are both wrong in exactly the same way?

 

A safer way to fill up when the gas pump has vapor guards is to pump normally while simply holding back the fuel nozzle seal so that the vapor guard system (pressure build up) doesn't kick in too early. That will allow it to fill the tank full and shut off as normal.

 

Not normally, but the drive was 442 miles, which was really stretching the limits of the MPG factor, so I took the 2-gallon can in order to add a safety factor without having to make time to off-ramp and go to a station. I did the fill at a rest stop while stretching my legs. I knew that almost 900 miles would be tough with these short days and didn't want to drive a new car at night without knowing the headlight setup, so cut as many corners as I could, including having the salesman have the new car sitting out front, full of fuel and air pressure 40 in the tires.

Left at 4AM, home at 6PM, a glass with two fingers of good scotch waiting on the table for me...it was a great day!

 

Also, for you guys that drive as slow as I do, is 20MPG about right for averaging 85? The C4 averaged about 23 averaging 75 (heavy fog)

 

I dunno man. Maybe you’re overthinking this a bit? If the car’s new to you (maiden voyage) maybe you just fill it when it’s empty and move on down the road. Figure out the quirks as you learn the car. I don’t know what you mean by “headlight setup”, but last time I did a longer drive like that in a new car I just took it slow, filled up when empty and turned on the lights when dark. Simple as that.

Also, heavy fog and 75? I don’t know where you live or how heavy the fog was, but whoa. New-to-you car, road trip, nerves and heavy fog (for me, at least) means slow down and take it easy.

Congrats on the new ride though. Get us some pics.

 

Agreed on maybe waiting to post questions until a month or two have passed, but hey, I'm old and time is getting short, lol. The fog part was driving over in the C4, and I'd had that for 4 months, so it was a Porsche without issues. The headlight thing is real....the C4 presents a wide, clear, bright scope across the field of view, while the C4S (pre-adjustments) gave a black box in front of the car, and the lights were like on a floating diaphragm whenever a bump was hit...but that's a different subject. Lighting is a big deal until I've adjusted them to how I want them to be.

The fuel thing is strange because right now I either have to believe that the gauges are correct and the tank holds only 14 gallons, or the tank holds 17 gallons and the gauges are weird. However, the only way I could have added 3 more gallons to the tank on the way back has to be explained. Did the dealer get the same FULL message while filling up, but only 14 got put into the tank, so while the car showed Full, it was really only at 3/4?
ull?
UU

 

Wayne partly explained the issue.
You have to fill the fuel tank to get a correct read on the gas gauge. It's in the 2007 owner's manual on page 103 under Level Gauge, "Note".
The tank has two parts, think of a walnut, you have to get above a certain level to fill the other half.

I get about 24 to 28 MPG, steady speed from 65 to 80 MPH.
Driving to work freeway and surface streets, its about 16-18 MPG.

Really, we didn't buy the C4S for gas mileage, did we?

Enjoy the drive, these cars are awesome.........

 

Today I'll try to add fuel to the tank with a gas can and see whether the tank will accept it (it should from all comments so far). If that's the case, I'll assume that the gauges said Full despite the lesser amount. The MPG question is to see how my car compares to others, mainly because a 14 year old car with 20K miles might need.....something....if what it's gauges are showing isn't normal (like a 225 degree oil temp being constant). Time to put this thread on the shelf and drive the car for a while, and report back if things continue.

 

My 06 C4s is sometimes off when filling up close to the last 1/8th in the tank ...Some times it fills up completely 17 gal other times it does not want more than 14 or 15 gal .. I found one pump near me that my car likes as some other pumps have slightly more output pressure and she no like. Also filling from empty does make the fuel gauge read off sometimes ..I found if i filled up from a quarter tank back to full I never have issues with the gauge.

 

Good reading, rileyracing1. Since how much to fill is really a moot point since I rarely let cars get below 1/2, it's important to know from a question of what works right, and what needs attention. I'll run several thousand miles and see what real MPG are, and compare to the fuel gauge, and go from there. Nowhere near a crisis, but more of a, "..are they fake, or real, and how can I know for sure?"